Lithium ion secondary batteries have been used in recent years as a power supply with ability of charge and discharge of electricity which is incorporated in cellular phones or portable electric devices. Moreover, for example, batteries using a solid-like unfluidized electrolyte as the electrolyte without danger of liquid leakage are also known. There are various outside forms of such a lithium ion secondary battery, and the batteries, generally used for notebook type or pocketbook type portable electric devices and cellular phones, are flat type in many cases. In such a flat type lithium ion secondary battery, in order to produce electric current continuity between a main body of a secondary battery cell and the exterior, board-like electrodes are respectively located on the positive pole/terminal and the negative pole/terminal.
The pole/terminal of positive electrode is generally formed with thin aluminum or an aluminum base alloy having a thickness of about 0.07–0.1 mm by the press cut or the like, in order to satisfy the structural limitations in a part connected to the main body of the secondary battery cell, restrain and reduce the thickness of the whole secondary battery, and ensure high conductivity, and a metal plate (the so-called tab) further connected with the exterior terminal lead is welded to the tip portion in many cases. As the metal plate, materials with low electric resistance, excellent mechanical strength, and excellent weather resistance such as a nickel base alloy are used suitably in order to electrically and mechanically (based on strength of material) ensure the connection with the exterior. Moreover, the plate thickness is set, for example, to 0.1 mm or more in many cases.
The metal plate and the electrode are generally welded by the electric resistance welding or the ultrasonic welding, and the sure welding is strongly needed so that the metal plate may not be separated nor omitted from the electrode, while using the secondary battery or the like.
However, there is a problem that poor welding occurs in the case of the electric resistance welding between the above electrode and the metal plate.
The first reason for the difficulty of the electric resistance welding is the difference between aluminum and nickel melting points. That is, the pole/terminal of positive electrode is formed with aluminum or the aluminum base alloy, so the melting point is 660–700° C. On the other hand, the metal plate is formed with the material with both high mechanical strength and a comparatively high melting point, so the melting point is, for example, 1400–1455° C. in the case of the nickel base alloy. Therefore, the difference between the electrode and the metal plate melting points reaches about 800° C. Moreover, the boiling point of aluminum is 2486° C., the boiling point of nickel is 2731° C., and the boiling points also differ greatly.
The second reason for the difficulty of the electric resistance welding is the existence of an oxide film (aluminum oxide) formed on the surface of the aluminum plate. The melting point of aluminum oxide is as high as 2050° C., and in the case of the electric resistance welding between the aluminum plate and the nickel plate, it is necessary to dissolve the thin oxide film under the temperature of the weld part of about 2050° C. or higher. Here, the aluminum oxide film on the aluminum surface is generally called alumina, and the chemical formula thereof is Al2O3.
Thus, if a welding condition is set up so that the welding temperature may reach the melting point or more of the metal plate made of the nickel base alloy or the like, in the case of welding a pile of one electrode and one metal plate by the conventional general electric resistance welding process, aluminum or the aluminum base alloy of the electrode dissolves completely over a large area to the extent that the plate thickness is penetrated exceeding the size of a normal/regular nugget. Accordingly, the dissolved metal is spilt out, aluminum evaporates and scatters around violently after the aluminum plate of the weld part reaches the boiling point, or the like phenomenon occurs, which causes generation of a hole in the part and poor welding without the sure welding.
Moreover, when the welding temperature is adjusted under the melting point of the metal plate, such as the nickel base alloy, in order to avoid such a complete broad dissolution of the electrode, the poor welding such as the hole generation in the electrode or the spill of the dissolved metal does not occur. However, since the nickel base alloy on the metal plate surface does not fuse, the normal nugget of both the nickel base alloy and the aluminum base alloy is not formed, only the trace of the aluminum base alloy melting is left on the cross section surface after the welding, and the poor welding of the unsure welding occurs. Consequently, for example, the peeling test reveals that the electrode and the metal plate separate easily under very lower power than the specific tensile proof stress.
Then, it is desired that various conditions of the electric resistance welding are set in order to generate a distribution of the welding temperature in which the normal nugget can be formed from the junction surface between the metal plate and the electrode to the peripheral part. However, such a condition setup unavoidably becomes very delicate, since the difference between the metal of the metal plate such as the nickel base alloy and metal of the electrode such as the aluminum base alloy melting points, is too large. Moreover, the tip of the electrode pole is degraded and deformed as the welding is continued, so it is very difficult to continuously maintain the setup of the above preferable delicate welding conditions in the actual mass production process, and therefore it is difficult to reduce the occurrence rate of the poor welding.
In addition, if the positive electrode terminal strip is composed of a nickel plate like the negative electrode terminal strip, the electric resistance welding can be carried out. However, if the nickel plate composes the positive electrode terminal strip, the nickel plate may react electrically in the inside of the battery and dissolve, and thus it is not preferable.
As described above, it is technically difficult to perform the electric resistance welding of the thin aluminum plate and the nickel plate, and particularly, the welding of the positive electrode terminal strip and a wiring board is conventionally performed by the ultrasonic welding.
However, there have been the following problems in the ultrasonic welding. First, the setting ranges of oscillating strength and amplitude time of the ultrasonic welding are small, and it is difficult to maintain the optimum conditions of the welding. Secondly, it is difficult to stabilize the welding strength, and the poor welding may occur at a certain rate in the mass production process for manufacturing the products in large quantities. Because, in the ultrasonic welding, only a very thin alloy layer is produced at the interface between the electrode and the metal plate, both are in the weakly connecting state only on the surface in many cases due to increasing the roughness of both the surfaces, and it is difficult to accomplish the demanded electrically and mechanically sure welding state for the welded surface. Moreover, the natural oxide film generated on the surface of the metal plate made of aluminum or the aluminum base alloy presents obstacles, and the weak welding action by the ultrasonic welding tends to be still weaker. And if the electrode and the metal plate are welded weakly as described above, the electric resistance becomes high in the part thereof, with such unfortunate consequences as the voltage which can be transferred from the secondary battery cell outside through the electrode and the metal plate decreases or the electric current can be limited.
Thirdly, ultrasonic welding devices are more expensive than resistance welding devices, and the equipment expenses for mass production becomes high. Fourthly, the ultrasonic welding devices are larger than the resistance welding devices, and a larger floor space is required. Then, developments in the technique for welding the thin aluminum plate and the nickel plate by the electric resistance welding have been demanded.
It is necessary, for example, to weld for long time using the supersonic wave with still stronger energy density or the like in order to produce the still stronger welding state by such ultrasonic welding. However, if the welding is performed with such strong supersonic wave for long time, the electrode, made of the aluminum base alloy with the thin thickness and the lower melting point, dissolves in the range exceeding the size required for welding like the case of the above electric resistance welding, and the advantages of the ultrasonic welding itself cannot be employed efficiently. For example, if too strong ultrasonic welding is performed, a part of the electrode made of the aluminum base alloy is cut.
Moreover, although the utilization of the soldering method is also considered as a method of electrically and mechanically fixing the metal plate to the electrode other than the welding, the natural oxide film (alumina) as described above is generated on the surface of the electrode made of the aluminum base alloy, and therefore presents obstacles to the wettability and attachment of the solder, and the soldering becomes difficult. It is thought that the treatment of pre-applying powerful flux or the like to the surface of the electrode for processing the natural oxide film of the surface before the soldering is effective to overcome the above obstacles. However, the inevitable result is that the component of such a strong flux remains on the electrode or the metal plate after soldering, so there are unfortunate consequences that the remaining component may remarkably degrade the durability of the connection part between the electrode and the metal plate. For example, the remaining components may gradually corrode the electrode during the period of using the secondary battery, which would eventually be damaged, dropped out, or the like. Moreover, soldering the positive electrode terminal strip and the wiring board is not preferable, since the thin aluminum plate composing the positive electrode terminal strip is brought to high temperature, and the inside temperature of the battery also becomes high, resulting in the battery degradation.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide a method of manufacturing a battery comprising the step of welding an electrode made of aluminum, an aluminum base alloy, or the like, and a metal plate made of a nickel base alloy or the like, in a polymer lithium ion secondary battery or the like, by means of an electric resistance welding process comprising an electric resistance welding step of securely welding the electrode and the metal plate, with eliminating the problems of poor welding such as hole generation or scattering to the surroundings due to the spilled metal in a welding part, and a battery with high reliability and durability in which a metal plate and an electrode are securely welded by such method.
It is another object of the invention to provide a method of manufacturing a weldment in which two or more objects being welded which made of a different material are easily welded by the electric resistance welding with high reliability, and a pedestal used for the method.